Receivers that detect stereophonic/monophonic signals can be incorporated into a vast number of devices used in everyday life. For example, such receivers can be used in automobile radios, a variety of communication systems such as cellular telephones, and even children's toys. Unfortunately, many modern receiver systems can suffer from performance shortfalls, such as frequent switchover between monophonic and stereophonic modes due to noisy channel environments and false detection of stereophonic signals as monophonic due to rated maximum system deviation (RMSD) mismatch.
In frequency modulated (FM) systems, an information signal (multiplex signal) modulates a frequency of a carrier that is transmitted over the airwaves. The carrier is a constant frequency signal that has a frequency high enough to be transmitted effectively. For FM systems specifically, as the information signal varies with time so does the instantaneous frequency of the modulated signal. In a complex baseband domain, the transmitted FM signal can be represented by the equation:
  exp  ⁡      (          j      ⁢                          ⁢      2      ⁢      π      ⁢                          ⁢              F        Δ            ⁢                        ∫          0          t                ⁢                              mpx            ⁡                          (              τ              )                                ⁢                                          ⁢                      ⅆ            τ                                )  where the information signal is the multiplex signal mpx(t).
A bandwidth of the modulated signal is represented by an RMSD value. In the above equation, the RMSD is represented by FΔ. Generally, two different RMSD values are accepted in standards governing FM transmission, 75 kHz and 50 kHz. A majority of transmitting standards require an RMSD value of 75 kHz, however, an RMSD value of 50 kHz is still being used by many broadcasters.
As a result of transmitted signals having an RMSD value of either 75 kHz and 50 kHz, a receiver may receive either. However, the receiver may be set to receive at a different RMSD value than the RMSD value of the transmitted signal. In this situation, the transmitted signal RMSD value and the RMSD value of the receiver are mismatched, and a stereophonic/monophonic state of the signal can be falsely interpreted by the receiver. This can result in the listener being denied the stereophonic quality of the program that the service provider is transmitting on the airwaves.
In order to receive FM audio signals, be they music or any other type of information, a receiver must be robust enough to handle changes in the channel caused by noise. Generally, a pilot tone is transmitted as part of the multiplex signal that is used to modulate an FM carrier signal in order to indicate that a transmission is stereophonic. The energy of the pilot tone may fluctuate significantly in a harsh channel scenario. Also, if a receiver is receiving at an RMSD value that is different than a transmitted RMSD value (i.e., the RMSD values are mismatched), then the received signal including the pilot tone can be distorted. Thus, simply comparing the pilot tone energy, estimated at the receiver, against a predetermined threshold may cause the receiver to switch between monophonic and stereophonic mode incorrectly or too frequently, and thereby degrade the entertainment quality of the audio program delivered to the consumer.